1. Field of the Invention
This invention relates to densification of particles of a particulate porous fluorinated resin, by which the density of the resin is increased to a higher level.
2. Description of the Prior Art
In general fluorinated resins are poorly or slightly swellable by, or poorly or slightly soluble in, their starting monomer or monomers. Therefore, the particles of the particulate resin prepared by suspension polymerization or bulk polymerization are liable to be porous particles having a small bulk density, since the polymer particles undergo little volume shrinkage in the course of polymerization.
The polymers having a small bulk density possess defects in that their biting into processing machines upon their melt molding is inferior and extruding efficiency is low when they are used as molding materials, as well as in that their coating property is inferior and the coating film is thin so as to readily form pinholes when they are used as powder coating materials.
In order to remove these defects of the conventional porous particulate resins, some methods for densification of polymer particles have been employed. Examples of the conventional methods include;
A. additional incorporation of monomer in the course of polymerization reaction, PA1 B. sintering of porous powder particles by means of hot gas stream, and PA1 C. freeze-milling of melt-pelletized resins.
The method (a) comprises polymerizing additional monomers in the pores of porous resin particles which were obtained by suspension polymerization, as disclosed in Japanese Patent Publication Nos. 14465/1971 and 3588/1971. This method, however, may have defects in that upon closure of the pores of particles as the polymerization proceeds, absorption velocity of the monomer is lowered and a long period of time is thus required to complete the polymerization.
The method (b) results in the considerable aggregation of resin particles, and the yield of particulate resins free from aggregates is as low as 20 - 30% after the densification. Therefore, it is not industrially advantageous.
The method (c) also has many defects such as poor extruding propety of the resins which are to be pelletized, high costs in freeze-milling and sieving steps and the like, indeterminate shapes of the resulting high density particles, and very broad range of grain size distribution.